However, as day broke over the sprawling encampments, police and army troops were nowhere to be seen.

Security sources told the Associated Press that the plan had been halted after it was leaked to the media. The postponement will temporarily allay concerns that a police operation could lead to mass casualties and deaths.

Pro-Morsi demonstrators have been occupying the Rabaa-el-Adaweya Square and a location outside Cairo University, since July 3 when military forces ousted Egypt's first democratically elected president. They say they will not leave until he is reinstated.

Threats to the sit-ins had raised fresh fears of bloodshed in a country that has grown dangerously polarised since Mr Morsi's removal. More than 300 people have died in political violence, with fierce clashes taking place between the Islamist president's supporters and opponents, and the country's security forces.

In an attempt to minimise the likelihood of fresh violence, police planned to surround the camps to prevent newcomers from entering. Shipments of food and water could also be cut off.

The remaining protesters would then be likely to face tear gas, water cannon and birdshot pellets.

Human rights groups have argued that this new push would also likely yield mass casualties. The encampments are densely packed, often with large families, and crowds regularly swell into the tens of thousands.

"Any police deployment around the sit-in will increase the likelihood of violent scuffles, even if they just cordon off the area." said Heba Morayef, Human Rights Watch Egypt director. "The gradual approach sounds good in theory, but we don't have a police force that knows how to use force proportionally and necessarily."

Two previous attempts to clear the sit-ins led to violent clashes between demonstrators and the security services, leaving more than 140 dead. If a similar display of force is used this time around, Ms Morayef said she feared the largest death toll ever seen in a single incident in Egypt.

The encampments remain a thorn in the side of Egypt's new leaders, as it attempts to move the country towards presidential elections early next year. Both state authorities and the Egyptian media have regularly portrayed the demonstrators as terrorists and a threat to national security, and front-page allegations of Muslim Brotherhood violence have become a near-daily occurrence.